Field
The invention relates to a capacitive micromechanical sensor structure. The invention relates also to a micromechanical accelerometer comprising two capacitive micromechanical sensor structures.
Description of the Related Art
In-plane acceleration detection in MEMS accelerometer is usually done with closing gap combs (parallel plate capacitor) or with linear combs (longitudinal capacitor).
In closing gap combs the capacitance behavior of the comb fingers can be approximately modeled by a parallel plate capacitor with varying finger gap and constant area according to equation (1)
                    C        =                  ∈                                    A                              d                -                x                                      +                          C              f                                                          (        1        )            where C is capacitance, ε is permittivity, A finger area, d finger gap, x finger displacement and Cf static stray capacitance. Comb fingers move in measurement mode closer to each other eventually causing the gap between the fingers to close.
In linear combs comb fingers move parallel to each other and capacitance behavior can be modeled by a parallel plate capacitor with varying area and constant finger gap according to equation (2)
                    C        =                  ∈                                                    h                ⁡                                  (                                      l                    +                    x                                    )                                            d                        +                          C              f                                                          (        2        )            where l is finger overlap length and h finger height.
Closing gap combs and linear combs have very different advantages and disadvantages. A closing gap comb can produce larger signals but a small gap between fingers makes the signal very nonlinear and it is hard to make an accelerometer that fulfills modern accelerometer requirements with finger side gaps under 2 micrometers. Also a process has to be able to manufacture a stopper gap with significantly smaller gap size than the measurement gap. Small finger side gaps cause reliability issues as electrical forces and mechanical sticking forces become close to returning spring forces. A linear comb, on the other hand, has smaller sensitivity with the same displacement, but the signal is linear. Linear combs also do not have pull-in effect in measurement direction and therefore sticking issues can be minimized.